Utility grade wind turbines (i.e., wind turbines designed to provide electrical power to a utility grid) can have relatively large rotors (e.g., 30 meters or more in diameter) and a rotor hub height that exceeds 100 meters. The extreme loads and fatigue stresses placed on the rotor blades of these turbines can be quite significant. Forces such as horizontal and vertical wind shears, yaw misalignment, turbulence, and the like, generate considerable bending of the blades that contributes to the total stresses placed on the blades. A critical design consideration in this regard is the joint between the blade root and the rotor hub.
A conventional joint design utilizes circumferentially spaced blade bolts to secure the blades to the rotor hub. A fundamental design consideration with respect to the bolts is to have a relatively flexible bolt and stiff flange so as to increase fatigue strength of the bolt. It is estimated that from about 40% to 80% of the total stress applied to the bolts is fatigue bending stress. These conventional bolts typically have threaded end sections that engage in threaded bores in the blade root flange and rotor hub flange, respectively. A more advanced wind blade bolt design utilizes a reduced diameter shank portion intermediate of the threaded end sections to increase the axial and bending flexibility of the bolt away from the thread stress concentration. However, this design also reduces the extreme load strength of the bolt due to the small shank diameter.
Accordingly, the industry would benefit from a blade bolt design that reduces bending or fatigue stresses without sacrificing the overall extreme load strength of the bolt.